[session] Ensure access to rql varmaker always mark the session as dirty
When one accesses the rql_varmaker, that's usually to use it. The pb was that when
the varmaker was already in page's data (which is stored as session data),
session storage such as redis won't see that the session data is dirty and has
to be stored back at the end of the request.
To fix this, systematically call set_page_data.
[native] Use cnx.system_sql instead of doexec method in syntax_tree_search
The latter handles connection issues and tries to reconnect. Method
syntax_tree_search appears to be called from quite a few place where it seems
useful to be resilient to connection issues (authentication for instance).
This reconnection logic appears to have disappeared at some point when working
on https://www.cubicweb.org/2919309. Could not find the exact reason though...